a. Field of the Invention
The present invention relates to an apparatus and method for liquid removal for wood pulp, and more particularly to such an apparatus and method which is particularly adapted for dewatering and/or washing material such as wood pulp or the like, and in a preferred form, both dewatering and washing.
b. Background Art
in the pulp and paper industry, there are in general three common methods of accomplishing a dewatering and/or washing operation. One method is to employ a rotating drum which has a perforate cylindrical sidewall, where the cylindrical sidewall on one side travels downwardly into a bath of a pulp slurry and then Travels upwardly to a location above the pulp slurry bath. A suction is applied within the drum, so that a portion of the pulp slurry adheres to the surface of the drum. As the layer of wood pulp on the drum is carried upwardly above the pulp slurry bath, in a first path of travel a dewatering operation is accomplished where the water from the pulp slurry is drawn into the interior of the drum. Then in a second part of the travel of the pulp on the drum, a wash water is deposited on the pulp mat that has been dewatered to cause displacement washing to be accomplished. Before the layer of pulp material that has been dewatered and washed travels back into the pulp bath, this layer is removed from the drum by a doctor blade or the like. The drum is sometimes enclosed in a pressure chamber, and for practical reasons, the pressure differential used in the drum-type dewatering/washing operation is in the range of about four to ten pounds per square inch.
A second method is to use a continuously moving foraminous conveyor belt onto which a wood pulp slurry is deposited. The conveyor belt carries the wood pulp slurry sequentially over a series of suction boxes which create a lower than atmospheric pressure below the belt to apply a differential pressure across the moving conveying belt to perform first a dewatering operation, and then a washing operation where wash water is deposited on the layer of wood pulp. In this type of operation, the pressure differential that can be applied across the pulp layer is limited because of the frictional force created between the moving belt and its underlying support structure, and the pressure differential limitations in such devices are generally in the range of about two to three pounds per square inch.
A third type of dewatering/washing operation is to move the pulp between upper and lower foraminous belts which are pushed toward one another to squeeze the water from the pulp. Then the pulp is mixed with a cleaning liquid, and the liquid removal operation is again repeated by again squeezing the pulp. This series of steps is continued until the desired dewatering and washing is accomplished.
To the best knowledge of the applicant, all of the dewatering/washing systems that have actually been used commercially operate with pressure differentials in the range of two to ten pounds per square inch, but no higher. As will be discussed later herein, the pressure differentials in the present invention are substantially higher, even up to three hundred PSI or higher.